1. Field of the Invention
The present invention relates to a knee voltage detector for a flyback converter, and more particularly, to a knee voltage detector capable of comparing an auxiliary related voltage of an auxiliary winding and a delay signal thereof on a primary side, to accurately detect a knee voltage.
2. Description of the Prior Art
A switching power converter can convert high voltage AC or DC power into low voltage DC power, and is widely used in all kinds electronic devices as a power supply. Common structures of the switching power converters include a flyback structure, a structure forward structure and a Push-Pull structure.
In general, a power converter can adjust a duty of a pulse width modulation (PWM) by detecting a magnitude of an output voltage, to generate the output voltage stably. Methods of detecting the output voltage can be divided into a secondary side feedback control structure which directly detects the output voltage and a primary side feedback control structure which indirectly detects the output voltage. On the primary side feedback control structure, a transformer of a power converter includes a primary side winding and a secondary side winding, and also includes an auxiliary winding on a primary side, and does not have to include an optical coupler and a three-terminal shunt regulator on a secondary side. When a current flows through the secondary side winding, the auxiliary winding can by induced by variations of the output voltage of the power converter. Therefore, a PWM control unit of the power converter can generate a feedback signal according to a voltage signal on the auxiliary winding, and then generates a control signal accordingly, to control a duty of a transistor and thus adjust electrical power for a load. Since the optical coupler and the three-terminal shunt regulator require high cost and large circuit area, the primary side feedback control structure can effectively reduce cost of the power converter.
In detail, please refer to FIG. 1A, which is a schematic diagram of a primary side control flyback circuit 10. As shown in FIG. 1A, compared with a secondary side feedback control structure, the primary side control flyback circuit 10 does not include a feedback network on a secondary side for directly detecting an output voltage VOUT, and thus needs to detect an auxiliary voltage VA of an auxiliary winding NA on a primary side (this structure detects an auxiliary related voltage VAUX to observe the auxiliary voltage VA, and the auxiliary related voltage VAUX is a divided voltage of the auxiliary voltage VA), to indirectly detect the output voltage VOUT, so as to generate a control signal VGD to control a duty of the transistor 100 for outputting the voltage VOUT stably.
Under this structure, please refer to FIG. 1B, which is a schematic diagram of signals of the primary side control flyback circuit 10 shown in FIG. 1A. As shown in FIG. 1A and FIG. 1B, after the control signal VGD controls the transistor 100 to turn on to charge a primary side winding NP and a secondary side winding NS for a charging period TON, the control signal VGD controls the transistor 100 to turn off and enter a discharging period TOFF. At this moment, via electromagnetic induction of the transformers (i.e. the auxiliary winding NA and the secondary side winding NS) the auxiliary voltage VA of the auxiliary winding NA can be induced by a secondary side voltage VS of the secondary side winding NS to observe information about the output voltage VOUT (in this structure, a primary side control circuit 102 detects the auxiliary related voltage VAUX of a divided voltage of the auxiliary voltage VA to observe information about the auxiliary voltage VA and the output voltage VOUT, wherein the auxiliary related voltage VAUX is equal to the auxiliary voltage VA multiplying a resistor ratio k of a divided voltage resistor).
However, the auxiliary voltage VA is interfered by many non-ideal factors, and cannot correctly reflect the output voltage VOUT. For example, in a voltage across a diode D1 on the secondary side varies with a discharging current IS on the secondary side (i.e. a falling effect), and a resonant effect occurs at the moment that the discharging current IS on the secondary side falls to 0. Therefore, in order to accurately detect the output voltage VOUT, the primary side control circuit 102 detects a knee voltage VC of the auxiliary voltage VA at the moment that the discharging current IS on the secondary side falls to 0, and then adjusts the output voltage VOUT accordingly via the control signal VGD (at this moment, the knee voltage VC is not interfered by the above falling effect and resonant effect.
For example, please refer to FIG. 2, which is a schematic diagram of operations of a conventional detector of the primary side control circuit 102 shown in FIG. 1A. As shown in FIG. 2, for a detector of U.S. Pat. No. 7,859,862, in order to prevent the interfered auxiliary voltage VA from being stored by a sampling circuit, a blanking circuit generates a blanking period first, to prevent the signal interfered by non-ideal factors when power elements are switched from being sampled. In such a situation, after the blanking period (from a time point A to a time point B), since the auxiliary voltage VA has a significant slope variation at the moment that the discharging current IS falls to 0 and a voltage at the moment can be sampled as the knee voltage VC, a differential circuit detects a slope variation of the auxiliary voltage VA, and a sampling circuit samples a delay voltage Vd of the auxiliary voltage VA to obtain the knee voltage VC when the slope variation exceeds a threshold value, wherein a delay time between the auxiliary voltage VA and the delay voltage Vd can be designed as a reacting time of the detector (i.e. from a time point of detecting a slope alteration exceeding the threshold value to a time point C of sampling the delay voltage Vd), to accurately sample the voltage at the moment that the slope alteration of the auxiliary voltage VA exceeds the threshold value as the knee voltage VC.
However, since a slope of the auxiliary voltage VA is effected by elements outside the system, accuracy of the detecting system is effected by external elements, and thus the system cannot be widely applied (e.g. a discharging slope of the discharging current IS is effected by the output voltage VOUT and the load and thus effect the falling slope of the auxiliary voltage VA, such that different conditions have different slope variations at the moment that the discharging current IS falls to 0).
On the other hand, please refer to FIG. 3, which is a schematic diagram of operations of another conventional detector of the primary side control circuit 102 shown in FIG. 1A. As shown in FIG. 3, for a detector of US 2007/0103134, a multi-sampling method is applied to repetitively sample the auxiliary voltage VA. When a target of the knee voltage VC occurs, a sampling result is outputted. In detail, a blanking circuit generates a blanking time Td first, and the detector does not perform detection during the blanking time Td, to avoid sampling signal affected by non-ideal factors. After the blanking time Td, a counter sends out the sampling signals VSPN VSP1 according to a sampling frequency, to control a sampling circuit to sample the auxiliary voltage VA accordingly until a sampled voltage of the auxiliary voltage VA have a significant variation, and then take a previous sampled voltage as the knee voltage VC.
However, accuracy of the above detector of degree is significantly limited by the sampling frequency. In order to increase system accuracy, the sampling frequency needs to be increased, thereby increasing power consumption and inducing electromagnetic interference.
Therefore, the conventional secondary side feedback control structure includes the optical coupler and the three-terminal shunt regulator, and thus requires high cost and large circuit area, while accuracy of the conventional primary side feedback control structure is effected by external elements otherwise the sampling frequency needs to be increased, thereby increasing power consumption and inducing electromagnetic interference. Thus, there is a need for improvement of the prior art.